This invention relates to fuel cell power systems and, more particularly, to a method for processing defective cells in fuel cell stacks.
In general, a fuel cell power system comprises one or more fuel cell stacks each including of a number of individual fuel cells assembled into a stack together with bipolar plates placed between neigboring two cells. To maintain the fuel cell stack at its optimum operating temperature, cooling plates are arranged every several fuel cells in the stack.
In such fuel cell stacks, if one or more fuel cells become defective or inoperative, they lower the efficiency of the overall fuel cell stack and have adverse influence on the fuel cells adjacent to the defective cells by their abnormal heat generation, resulting in shortening the lifetime of the stack. In order to avoid such problems, it is the best way to replace the defective cell with new one every time one fuel cell becomes defective. However, the replacement of the individual defective cell is very troublesome and takes a long time since the fuel cells are electrically connected in series and physically assembled into a stack.
To solve such problems, it has been proposed in U.S. Pat. No. 4,397,918 (corresponding to Japanese patent application Laid-open No. 58-184772) to shortcircuit a particular set of fuel cells which contains one or more defective cells. In the prior art, the shorting is carried out, for example, as shown in FIG. 7, by inserting conductors 5 into respective cooling gas passages 4 to connect successive cooling plates 3 which sandwich a particular set of the fuel cells containing one or more defective cells. This method is readily carried out and downtime of the stack is greatly minimized.
However, in such a fuel cell stack, several cells, for example, five cells are sandwiched by successive cooling plates, so that, if only one fuel cell become inoperative or defective, the remaining four fuel cells are bypassed by the shortcircuit path. Thus, this process has a great influence of the performance of the overall fuel cell stack.
It is therefore an object of the present invention to provide a method for processing defective cells in fuel cell stacks which makes it possible to shortcircuit the defective cells only with ease and certainty, without having any influence on the fuel cells adjacent to the defective fuel cells.